Update changelog for v3.28.21

Author: mbg

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 See the [releases page](https://github.com/github/codeql-action/releases) for the relevant changes to the CodeQL CLI and language packs.
 
+## 3.28.21 - 28 July 2025
+
+No user facing changes.
+
 ## 3.28.20 - 21 July 2025
 
 - Remove support for combining SARIF files from a single upload for GHES 3.18, see [the changelog post](https://github.blog/changelog/2024-05-06-code-scanning-will-stop-combining-runs-from-a-single-upload/). [#2959](https://github.com/github/codeql-action/pull/2959)